1. Field of the Invention
This invention relates generally to protective relay apparatus for protecting ac electrical power transmission lines, and more specifically, to pilot protective relay apparatus for providing current differential protection on a multi-terminal ac electrical power transmission line.
2. Description of the Prior Art
Electrical power transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits, which could cause collapse of the power system and serious and expensive apparatus damage. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a fault condition, line currents can increase to several times the normal value thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in approximately 0.1 to 0.5 seconds. This isolation time limit must allow for the operation of large circuit breakers, interrupting up to 80,000 A, and the completion of back-up operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which monitor ac voltages and currents at the transmission line terminals, to locate faults and initiate line isolation via tripping of the appropriate circuit breakers. These faults can be located using any one of several different techniques based on the relationship of the ac voltages and/or currents at the transmission line terminals.
Pilot protection for high-voltage electrical transmission lines identifies faults on the protected line segment (i.e., that section of transmission line with protective relays located at each end) by providing means for the protective relay at each terminal of the protected line segment to receive information regarding system conditions at the other terminal (i.e., the remote terminal). Each protective relay, using the local and remote information, can then make a decision regarding the condition of the intervening protected line segment. For economic reasons, power-line carrier communications or microwave pilot channels are used for long transmission lines, and pilot-wire relaying is used for protecting short transmission line sections of about 10 miles or less. For short high-voltage transmission lines, pilot-wire protective relaying is the only practical technique not based on a time delay, because discrimination with distance-type relays is difficult over such a short distance.
The earliest type of pilot-wire relays required a continuous metallic circuit between the terminals of the protected line segment. In this prior art system, a composite sequence filter at the local and remote terminals converts the three phase currents into a single voltage. The voltage is then applied to the pilot wire, so that the local and remote voltages can be compared at each terminal. These protective relays are electromechanical, including the well-known operating and restraint coils. For normal conditions, more current flows in the restraint coils due to the polarities of the local and remote voltages developed by the composite sequence filters. Under fault conditions, one of the voltages experiences a phase-angle shift, causing more current to flow through the operating coils, tripping the relay and opening the circuit breaker.
U.S. Pat. No. 4,275,429, which is assigned to the same assignee as the present invention, discloses a new and improved current differential pilot relay providing the functions of an electromechanical pilot-wire relay without the necessity of having a continuous, metallic conductor between the local and remote protective relays. That is, this improved pilot-wire relay can use a noncontinuous communications channel, such as a low-grade, uninterruptible dedicated telephone line, an optical link, a microwave channel, or power line carrier. A dedicated telephone line maintains an open channel at all times, but has relatively low current capability and is not a continuous metallic circuit, as amplifiers and switchboards may be included in the circuit.
This prior art protective relay system is an all solid-state system, including a protective relay at the local and remote terminals of the protected line segment for comparing voltage signals representative of the current at the local and remote terminals. A composite sequence filter at each terminal develops the voltage signal based on the three phase currents at that terminal. At each terminal, a modulated signal representative of the current-derived voltage signal, is transmitted to the other terminal. At the receiving terminal the modulated signal is demodulated to reproduce the current-derived voltage signal for comparison with the local current-derived voltage signal. Pulse period modulation is a preferred form of communications because the filtering requirements in the demodulator are miniscule, but frequency modulation or any other suitable communications scheme may be used. Additional details of suitable modulation and demodulation techniques can be found in commonly assigned U.S. Pat. No. 4,380,746 and Application Ser. No. 397,944 (filed July 13, 1982), respectively.
An undesirable complexity arises from use of this prior art current differential pilot relay on a multi-terminal transmission line. Assuming a three-terminal line, a protective relay of the prior art solid-state type would be located at each terminal. In one arrangement, communications channel connecting these three protective relays is arranged such that signals from the first terminal propagate to and through the second terminal to the third terminal. Likewise, signals from the third terminal propagate to and through the second terminal to the first terminal. This prior art arrangement requires a total of six receiver-transmitter combinations. The first terminal includes a single transmitter to transmit the pulse-modulated signal to the second terminal. The first terminal also includes two receivers, one each for receiving a signal from each of the second and third terminals. The third terminal also includes a single transmitter and two receivers. The transmitter transmits the current-derived voltage signal from the third terminal to the second terminal, and the two receivers receive the current-derived voltage signals from the first and second terminals. The arrangement at the second terminal is more complex; it includes four transmitters and two receivers. One receiver receives the pulse period modulated signal from the first terminal, and the other receives the pulse-period modulated signal from the third terminal. The first transmitter transmits the signal from the third terminal to the first terminal, and the second transmitter transmits the local signal from the second terminal to the first terminal. The third transmitter transmits the signal from the first terminal to the third terminal, and the last transmitter transmits the local signal from the second terminal to the third terminal.
At each terminal the received signal is demodulated to reproduce the current-derived voltage signal corresponding to the current at each of the other two terminals. At each terminal the local signal is compared with the two received (or remote) signals for fault detection. This technique of comparing the local and remote signals at each terminal allows tripping to be initiated at high speed at all the terminals for faults on the protected transmission line. The primary disadvantage of this prior art technique is the expense associated with the use of transmitter/receiver combinations to transmit to all terminals from all terminals. The present invention discloses a technique for reducing the number of transmitter-receiver pairs and the corresponding cost associated with the prior art protective relay system.